clen dosage

My sister is taking Clen for weight loss...she's about 5'7" 135lbs. I belive she is taking 3 pills a day, I'm not sure of the dose, but I"m sure its the standard dose (maybe 20 or 30 mgs)...she's been taking for 3 weeks and no results, says she doesent even feel a difference...any suggestions for clen use in this situation...